Microstrip rf variable attenuator

ABSTRACT

A microwave RF variable attenuator with a microstrip arcuate attenuator section in a transmission line conductor of the microstrip bonded to a relatively low dielectric constant material (polyolefin) in turn bonded to an electrically conductive metal ground plane plate. A relatively high dielectric low loss circular disc mounted over the arcuate attenuator section of the transmission has an eccentrically smaller diameter thin lossy film of tantalum metal approximately 2 microinches thick positionable into greater or lesser overlying relation to the arcuate attenuator section of transmission line to increase and decrease RF signal attenuation thereby through the transmission line.

United States Patent 51 Apr. 25, 1972 Hallford [s41 MICROSTRIP RF VARIABLE ATTENUATOR {72] inventor: Ben R. l-lalllord, Dallas, Tex. [73] Assignee: Collins Radio Company, Dallas, Tex.

- [22] Filed: July 8, 1970 [2]] Appl. No.: 53,054

[52] US. Cl. .l...333/8l A, 333/84 M [51] lnt.Cl. ..H0lp l/22 [58] Field of Search ..333/8l, 8! A, 84 M [56] References Cited UNITED STATES PATENTS 2.96l,62l ll/l960 Tanenbaum et al. ..333/8l A 3,1 l9,08l l/l964 Jordan ....333/84 M 2,810,891 l0/l957 E ngelmann ..333/8l A 2,909,736 10/1959 Sommers et al "333/81 A OTHER PUBLICATIONS Von Hippel; Dielectric Materials and Applications, M.l.T.

Press, Cambridge, Mass. 1954, pp. 303, 332, 335, 340 cited, QC585V6 Primary Examiner-Paul I... Gensler Attorney-Warren H. Kintzinger and Robert J. Crawford ABSTRACT A microwave RF variable attenuator with a microstrip arcuate attenuator section in a transmission line conductor of the 'microstrip bonded to a relatively low dielectric constant material (polyolefin) in turn bonded to an electrically conductive metal ground plane plate. A relatively high dielectric low loss circular disc mounted over the arcuate attenuator section of the transmission has an eccentrically smaller diameter thin lossy film of tantalum metal approximately 2 microinches thick positionable into greater or lesser overlying relation to the arcuate attenuator section of transmission line to increase and decrease RF signal attenuation thereby through the transmission line.

11 Claims, 3 Drawing Figures PATENTEDAPRZS m2 :3. 659.233

'mvzuron. ac-lv n. HALLFORD ATTO MICROSTRIP RF VARIABLE ATTENUATOR This invention relates in general to RF variable attenuators, and in particular, to an RF microwave microstrip attenuator continuously variable with adjustment thereof while maintaining substantially constant RF impedance throughout the entire range of attenuation variation.

There has been a long standing need for a variable RF attenuator particularly with RF microwave microstrip circuitry with the attenuator continuously variable, having a low insertion loss when at a minimum loss position and with substantially constant impedance throughout the range of attenuation adjustment. Many previously used attenuating adjustable attenuation devices have employed resistance cards made with a carbon composition placed on an epoxy fiberglass sheet usually approximately one-sixteenth inch thick or more for rigidity. Some other more recent'attenuating devices have used a metalized film deposited on epoxy fiberglass. Generally, these materials such as epoxy fiberglass and other such supporting structural materials used with resistive films have, for example, a dielectric constant of approximately four and are themselves lossy to RF signals. Further, such attenuators have been usually curved in order to reduce abrupt transitions on the RF conductor in order that their movement across the RF conductor would cause the least amount of discontinuity. Even if this transition were perfect, the impedance of the conductor generally has been drastically altered by the relatively high dielectric constant of the attenuator structural supporting material to which the resistive film is bonded and with this change of impedance becoming progressively worse as attenuation is increased. Various other attenuating devices have used plastic materials such as mylar to support a resistive film with the mylar not being lossy by itself but with thin sheets of such plastic material being troublesome in not providing the degree of mechanical rigidity actually desired.

It is, therefore, a principal object of this invention to provide an RF microwave attenuator for use with microstrip that maintains substantially constant RF impedance throughout the entire range of attenuation settings.

Another object with such an attenuator is to keep ceramic over the same conductor area as the attenuation setting is varied and to retain substantially the same line characteristic impedance under the ceramic as with open exposed stripline conductors.

A further object is to minimize mismatch created by the presence of the lossy thin-film material over the microstrip attenuator circuit section.

Features of the invention useful in accomplishing the above objects include, in an RF microwave microstrip attenuator, microstrip thin strip circuitry bonded to a relatively low dielectric constant dielectric material layer in turn bonded to a metal ground plane plate with an exposed microstrip transmission line having an attenuator section. This transmission line attenuator section is formed with an arcuate section with opposite ends thereof connected in smoothly curved transition end connective sections to respective opposite transmission line portions. The portion of the transmission line forming the attenuator section thereof has lessened conductor line width in order to retain the same impedance therethrough under ceramic material of the attenuator device since the overlying ceramic material effectively lowers conductor impedance by increasing the capacity of the conductor to ground. The attenuator structure also includes this ceramic material in the form of a circular uniformly thick relatively thin ceramic disc with a face supported in intimate contact with the attenuator section of the microstrip circuitry mounting a smaller diameter circular eccentrically positioned relatively thin approximately 2 microinches thick lossy tantalum film thereon. This eccentrically positioned circular lossy thin film may be positioned and locked in various adjustment positions from a minimum position to a maximum attenuation position to attain different RF microwave attenuation settings as may be desired.

A specific embodiment representing what is presently regarded as the best mode of carrying out the invention is illustrated in the accompanying drawing.

In the drawing:

FIG. 1 represents a top plan view of a section of microstrip with an arcuate transmission line attenuator section and a relatively rotationally positionable attenuator lossy film carrier in phantom thereabove;

FIG. 2, an elevation sectioned view substantially on line 2 2 of FIG. 1 with detail of both the microstrip section and the attenuator lossy film carrier mounted thereon; and

Figure 3, a view of the circular bottom of the attenuator lossy film carrier with the lossy film an eccentrically positioned circular film bonded thereon.

Referring to the drawing:

In the section of microstrip 10 of FIG. 1 thin strip circuitry 11 is bonded to a relatively low dielectric constant dielectric material layer 12 having a dielectric constant in the range of approximately 2 to 4, that is a 'lower dielectric constant material than encountered with ceramic or other such substrates. The lower dielectric constant material 12 is a material such as polyolefin plastic laminate bonded to a metal ground plane plate 13, as shown in FIG. 2. The thin strip circuitry 11 shown includes an arcuate transmission line section 14 that falls under a ceramic disc 15, indicated in phantom in FIG. 1, that is mounted for rotation about a center concentric with the arcuate center of reference for the transmission line arcuate section 14. Please note that the transmission line circuitry 11 undergoes a transition to lesser width under the ceramic disc 15. The conductor portion under the ceramic 15 must have a narrower line width to retain the same impedance since the ceramic material of the disc 15 lowers the conductor impedance by increasing the capacity of the conductor to ground, that is, to the ground plate 13 through the dielectric material 12. Please note with polyolefin dielectric having approximately a 2.3 to 2.5 dielectricconstant and approximately 0.026 inches thick, that one ounce (0.0014 inch thick) copper bonded to the upper side for the microstrip copper conductors provides 50 ohm line with a copper conductor width of approximately 0.080 inches through the lengths thereof not under the ceramic discl 5, and under the disc it is reduced to approximately 0.060 inches in width.

A relatively thin metalized film of tantalum that has been deposited by vapor deposition as an eccentrically located smaller diameter circle 16, also shown in FIG. 3, is deposited on the ceramic disc 15 to approximately 2 microinches thickness to provide a lossy material film with a resistivity factor of approximately ohms per square. The film is considered a lossy film in which eddy currents set up within the thin film are transformed to heat and dissipated as such. The ceramic disc is fastened to a backing and structural mounting device 17, made of plastic, by being bonded (detail not shown) to a large circular base 18, of device 17, also giving backing support to the relatively thin ceramic disc 15. The mounting device 17 also has an elongate hub with an opening 19 that is a relatively rotatable fit on a rod 20 that is mounted on the microstrip with a bottom threaded extension 21 threaded into threaded opening 22 of ground plane plate 13. Rod 20 is also provided with an upper threaded extension 23 receiving a plastic threaded lock member 24 for locking attenuation rotationally set positions of the disc 15 with the lossy film 16. Please note that a relatively small center conductor metal ring 25 is left about the center opening 22 and also an outer relatively short arcuate section 26 of thin strip conductive metal is left on the substrate 12 to provide support for the ceramic disc when it is locked in place in an attenuation set position on the microstrip board. These conductive metal pads lend support to the ceramic disc in strategic structural areas where no RF conductor is present including the center where pressure is applied to the disc by the mounting assembly. These pads support the disc evenly to avoid tilting or unequal stresses and permit the disc to be held firmly against the RF conductor in attaining the most attenuation and in avoiding undesired changes in the RFconductor impedance.

Thus, a constant impedance RF variable attenuator has been provided that is usable up to a frequency of 18 Gl-lz in units that have been built with attenuation attained by placing, and and adjusted by, moving a thin metalized film, over a conductor carrying RF energy. To reiterate, RF fields exist above a microstrip conductor with an intensity that is inversely proportional to the dielectric constant of the substrate and directly proportional to the substrate height. Use of relatively thin low dielectric constant substrates provides a sufficient field strength above the microstrip conductor to cause a loss of energy from lossy materials that may be placed in contact with the conductor. Please note that this attenuator structure is designed for operation only with open conductor type of striplines commonly called microstrip. With this being the case a thin film of tantalum deposited on a low loss relatively high dielectric constant disc is placed next to a transmission line conductor as the means for obtaining attenuation.

The low loss relatively high dielectric constant (over 6 8) disc in working units to date is in the form of a low loss ceramic disc with the thin deposited metalized layer and is in position for minimum RF attenuation when the attenuator is set to the minimumloss position shown in FIG. 1. When the quite rigid ceramic disc 15, that has good stability with temperature, is rotated approximately 180 from the position shown in FIG. 1, it would be set for maximum RF signal attenuation. Further, this is with constant RF impedance having been maintained by keeping the ceramic over the same conductor area as the attenuation setting is varied through rotation of the disc about its center to different settings. The metalized film is moved across the conductor as the ceramic disc is rotated and the lossy tantalum film surface edge is always at an angle to the conductor when it is moved into adjacency therewith and thereover to minimize the mismatch created by its presence. When the lossy film is moved from setting to setting, it is moved over more of the conductor area in a controlled manner allowing a gradual increase in the magnitude of the attenuation as the disc is constantlyrotated. In the minimum loss position of the ceramic disc with the eccentric circle of lossy film the lossy film should be sufficiently removed from the conductor as to cause substantially no loss to the RF field. In order that the operational behavior that has been described may be accomplished the metalized lossy film is deposited in the form of a circular pattern with a diameter less than that of the ceramic disc and with one edge of the circular film tangent to the edge of the ceramic disc. Further, a metalization area 27 of the thin film lossy disc is removed at the center of the ceramic disc to avoid metal to metal contact with the metal mounting post 20 and also the conductive metal support pad 25 lending support at the center to the ceramic disc 15. This is a precaution taken to avoid undesired change in RF impedance and to prevent any lossy film dc circuit connection to ground.

Whereas this invention is here illustrated and described with respect to a specific embodiment hereof, it should be realized that various changes may be made without departing from the essential contributions to the art made by the teachings hereof.

I claim:

1. In an RF microwave microstrip variable attenuator, a rigid electrically conductive ground plane plate mounting a dielectric material layer and exposed microstrip circuitry in laminate relation thereon; said dielectric material having a relatively low dielectric constant in the range of approximately two to four and bonded on a first side to said ground plane plate; said exposed microstrip circuitry bonded to the second side of said dielectric material and including a microstrip transmission line with an attenuator portion; said transmission line attenuator portion being formed with a substantially arcuate section; with opposite ends in the form of smoothly curved transition end connective sections to the respective connective opposite transmission line sections; a relatively rigid circular disc element of low loss relatively high dielectric constant material with a relatively fiat circular face; a, relatively, very thin lossy film bonded to the flat face of said circular relatively rigid element of dielectric material through an eccentrically positioned area portion over less area of said flat circular face than the area thereof; mounting means supporting said fiat circular face and the thin lossy film bonded thereto in close intimate contact with said transmission line attenuator portion in any rotationally set position of the flat face and lossy film over the attenuator portion; and wherein said microstrip transmission line is transitioned from a wider to lesser width through said smoothly curved transition end connective sections to said lesser width under said low loss relatively high dielectric constant circular disc to retain substantially the same impedance with the material of the circular disc lowering conductor impedance thereunder by increasing the capacity of the transmission line conductor thereunder to ground; and wherein said mounting means mounts said circular relatively rigid element of dielectric material for rotation in setting adjustment of said circular rigid element of dielectric material about a rotational center substantially common to the geometric-center for striking the arc of said attenuator portion substantially arcuate section; and with said substantially arcuate section being of said lesser width substantially throughout the entire length thereof.

2. The. RF microwave microstrip variable attenuator of claim 1, wherein said circular disc is low loss relatively high dielectric constant ceramic; and with microstrip conductors providing 50 ohm line being copper conductors with a width of approximately 0.080 inches through lengths thereof not under the ceramic disc, and with, under the disc, conductor width reduced to approximately 0.060 inches in width.

3. The RF microwave microstrip variable attenuator of claim 1, wherein said very thin lossy film is in the form of a circular area of film of smaller diameter than the circular relatively rigid element of dielectric material; and with the lossy film circular area eccentrically positioned on said flat circular face.

4. The RF microwave microstrip variable attenuator of claim 3, wherein said very thin lossy film is deposited to a thickness giving a resistivity factor of approximately ohms per square.

5. The RF microwave microstrip variable attenuator of claim 3, wherein said circular disc is a relatively thin uniformly thick circular disc .with a dielectric constant falling above approximately 8.

6. The RF microwave microstrip variable attenuator of claim 5, wherein said relatively thin uniformly thick circular disc is a rigid disc of ceramic dielectric material.

7. The RF microwave microstrip variable attenuator of claim 5, wherein said very thin lossy film is a thin metalized film of tantalum approximately 2 microinches thick.

8. The RF microwave microstrip variable attenuator of claim 5, wherein said dielectric material deposited on said ground plane plate is polyolefin dielectric having approximately a 2.3 to 2.5 dielectric constant.

9. The RF microwave microstrip variable attenuator of claim 5, wherein said mounting means includes a plastic material hub member with a bottom flange backing said circular disc; said hub member rotatable with said circular disc about a mounting rod fixed to said microstrip ground plane plate; and lock means locking rotation selected adjusted positions of said hub member and said circular disc and holding said circular disc and lossy film in intimate pressed substantially uniform contact with said attenuator portion of the microstrip transmission lines.

. 10. The RF microwave microstrip variable attenuator of claim 9, wherein microstrip circuitry thick padding support means is provided for supporting said circular disc in spaced non-circuit areas hereunder.

11. The RF microwave microstrip variable attenuator of claim 10, wherein-said padding support means includes a center annular ring pad, and an arcuate pad section; and a circular area of lossy thin film is removed from the area of said lossy thin film at the center of said circular disc. 

1. In an RF microwave microstrip variable attenuator, a rigid electrically conductive ground plane plate mounting a dielectric material layer and exposed microstrip circuitry in laminate relation thereon; said dielectric material having a relatively low dielectric constant in the range of approximately two to four and bonded on a first side to said ground plane plate; said exposed microstrip circuitry bonded to the second side of said dielectric material and including a microstrip transmission line with an attenuator portion; said transmission line attenuator portion being formed with a substantially arcuate section; with opposite ends in the form of smoothly curved transition end connective sections to the respective connective opposite transmission line sections; a relatively rigid circular disc element of low loss relatively high dielectric constant material with a relatively flat circular face; a, relatively, very thin lossy film bonded to the flat face of said circular relatively rigid element of dielectric material through an eccentrically positioned area portion over less area of said flat circular face than the area thereof; mounting means supporting said flat circular face and the thin lossy film bonded thereto in close intimate contact with said transmission line attenuator portion in any rotationally set position of the flat face and lossy film over the attenuator portion; and wherein said microstrip transmission line is transitioned from a wider to lesser width through said smoothly curved transition end connective sections to said lesser width under said low loss relatively high dielectric constant circular disc to retain substantially the same impedance with the material of the circular disc lowering conductor impedance thereunder by increasing the capacity of the transmission line conductor thereunder to ground; and wherein said mounting means mounts said circular relatively rigid element of dielectric material for rotation in setting adjustment of said circular rigid element of dielectric material about a rotational center substantially common to the geometric center for striking the arc of said attenuator portion substantially arcuate section; and with said substantially arcuate section being of said lesser width substantially throughout the entire length thereof.
 2. The RF microwave microstrip variable attenuator of claim 1, wherein said circular disc is low loss relatively high dielectric constant ceramic; and with microstrip conductors providing 50 ohm line being copper conductors with a width of approximately 0.080 inches through lengths thereof not under the ceramic disc, and with, under the disc, conductor width reduced to appRoximately 0.060 inches in width.
 3. The RF microwave microstrip variable attenuator of claim 1, wherein said very thin lossy film is in the form of a circular area of film of smaller diameter than the circular relatively rigid element of dielectric material; and with the lossy film circular area eccentrically positioned on said flat circular face.
 4. The RF microwave microstrip variable attenuator of claim 3, wherein said very thin lossy film is deposited to a thickness giving a resistivity factor of approximately 100 ohms per square.
 5. The RF microwave microstrip variable attenuator of claim 3, wherein said circular disc is a relatively thin uniformly thick circular disc with a dielectric constant falling above approximately
 8. 6. The RF microwave microstrip variable attenuator of claim 5, wherein said relatively thin uniformly thick circular disc is a rigid disc of ceramic dielectric material.
 7. The RF microwave microstrip variable attenuator of claim 5, wherein said very thin lossy film is a thin metalized film of tantalum approximately 2 microinches thick.
 8. The RF microwave microstrip variable attenuator of claim 5, wherein said dielectric material deposited on said ground plane plate is polyolefin dielectric having approximately a 2.3 to 2.5 dielectric constant.
 9. The RF microwave microstrip variable attenuator of claim 5, wherein said mounting means includes a plastic material hub member with a bottom flange backing said circular disc; said hub member rotatable with said circular disc about a mounting rod fixed to said microstrip ground plane plate; and lock means locking rotation selected adjusted positions of said hub member and said circular disc and holding said circular disc and lossy film in intimate pressed substantially uniform contact with said attenuator portion of the microstrip transmission lines.
 10. The RF microwave microstrip variable attenuator of claim 9, wherein microstrip circuitry thick padding support means is provided for supporting said circular disc in spaced non-circuit areas hereunder.
 11. The RF microwave microstrip variable attenuator of claim 10, wherein said padding support means includes a center annular ring pad, and an arcuate pad section; and a circular area of lossy thin film is removed from the area of said lossy thin film at the center of said circular disc. 